Introduction to Perl
October 4, 2004
Class Meeting 7
* Notes on Perl by Lenwood Heath, Virginia Tech © 2004
Perl Background
Practical Extraction and Report
Language (Perl)
 Created by Larry Wall, mid-1980's
 Language combining capabilities of shell
programming, awk, grep, lex, sed, and
a number of other UNIX utilities
 Powerful, complex scripting language
 We learn just a bit!

Lenwood Heath, Virginia Tech, Fall, 2004
2
Scalars
Basic data type in Perl is scalar
 Most scalar values are numbers or
character strings
 Programmer forces interpretation of a
scalar value as a number or string by
operations used
 Special scalar value undef is neither
number nor string, just "undefined"

Lenwood Heath, Virginia Tech, Fall, 2004
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Numbers
Integers: 45, 974, -892, 0
 Real numbers: 45.0, 10.237, -101.1,
2.5e-3
 Octal: 055
 Hexadecimal: 0x2d
 Binary: 0b101101

Lenwood Heath, Virginia Tech, Fall, 2004
4
Numeric Operators

Arithmetic: 4+5, 9-7, -9*-3, 10/3


Comparisons: <, >, <=, >=, ==, !=


Modulus (remainder): 102 % 9 is 3
Spaceship: <=> (-1, 0, or 1)
Logical
And
 Or
 Not

&&
||
!
and
or
not
Lenwood Heath, Virginia Tech, Fall, 2004
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Strings

Zero or more characters: "", "one"
No concept of null-termination
 Single (literal) quotes



tab\tnl\n
9
Double (interpreted) quotes


'tab\tnl\n'
"tab\tnl\n"
tab_nl_
7
Escaped double quote

"Here's a double quote \"."
Lenwood Heath, Virginia Tech, Fall, 2004
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String Operators
Concatenation: "Learning "."Perl"
 Comparisons: lt, gt, le, ge, eq, ne


Index: position of a substring in a string
index('Learning Perl','rni')
 index("Learning Perl",'nr')


Substring: select a substring


3
-1
substr('Learning Perl',1,2)
ea
String positions start at 0
Lenwood Heath, Virginia Tech, Fall, 2004
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Scalar Variables

Scalar variable identifier begins with $
$colors = "red green blue";
 $count = $count+1;


Shortcuts and alternatives:
$colors .= ' purple';
 $count += 1;
 $count++;


Interpolation: "Count is $count.\n"
Lenwood Heath, Virginia Tech, Fall, 2004
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Lists

Sequence of scalars


(5.7,"house\tbarn",'-9.2')
qw shortcut — equivalent lists:
("VT","UNC","NCSU","UVa","Wake")
 qw/ VT UNC NCSU UVa Wake /
 qw{ VT UNC
NCSU UVa
Wake
}

Lenwood Heath, Virginia Tech, Fall, 2004
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Arrays
An array is a list-valued variable
 Array identifier begins with @



@colors = qw(red green blue);
Array element reference: $id[index]
$colors[2]
# Value is 'blue'
 $colors[8] = 'purple';
 substr($colors[1],0,3) # 'gre'

Lenwood Heath, Virginia Tech, Fall, 2004
10
Simple Perl Script
#!/usr/bin/perl
$dotted = join('.',@ARGV);
@ping = `ping -c 1 $dotted`;
print @ping[0..1];
Result:
[cs2204@peach cs2204]$ dot_ping www cslab vt edu
PING owlstation.cs.vt.edu (128.173.40.52) 56(84)
bytes of data.
64 bytes from 128.173.40.52: icmp_seq=1 ttl=64
time=0.536 ms
Lenwood Heath, Virginia Tech, Fall, 2004
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Numeric Functions








ASCII code — ord('?') is 63
ASCII character — chr(63) is '?'
Absolute value — abs(-11) is 11
Integer value — int(295.143) is 295
Square root — sqrt(16) is 4
Natural logarithm — log(295.143) is 5.69
Integer value — int(295.143) is 295
Random number — rand(10) was 4.94028
Lenwood Heath, Virginia Tech, Fall, 2004
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String Functions
Length — length('Learning Perl') is 13
 Find substring — index
rindex
 Extract substring — substr
 Lower case — lc('9Jp.iR') is '9jp.ir'
 Upper case — uc('9Jp.iR') is '9JP.IR'
 Remove last character — chop($word);
 Remove newline at end — chomp($line);

Lenwood Heath, Virginia Tech, Fall, 2004
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Array Functions

Stack; top on the right — @stack=qw/1 2 3/;



Stack; top on the left — @stack=qw/4 7 a/;



push(@stack,'top') updates @stack to 1 2 3 'top'
pop(@stack) updates @stack to 1 2 3 returns 'top'
unshift(@stack,8) updates @stack to 8 4 7 'a'
shift(@stack) updates @stack to 8 4 7 returns 'a'
Reverse a list

Reverse qw/8 l a p 7/ returns qw/7 p a l 8/
Lenwood Heath, Virginia Tech, Fall, 2004
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Array Functions (Continued)

Array to string



String to array


split(/b/,$words) is qw/9 4--x.;--p j/
Sorting — lexicographic order


@words = qw/9b4 x.; pbj/;
$words=join('--',@words); is '9b4--x.;--pbj'
sort qw/red green blue/ returns qw/blue green red/
Sorting — numerical order

sort { $a <=> $b } (94,-1,55) returns (-1,55,94)
Lenwood Heath, Virginia Tech, Fall, 2004
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Input

Text files

Sequence of lines, each terminated by a newline

File access by a file handle — standard input
STDIN

Read a line


$line = <STDIN>;
Read remaining lines

@lines = <STDIN>;
Lenwood Heath, Virginia Tech, Fall, 2004
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Output

Standard output — STDOUT


print 'A line to', " standard output\n";
Standard error — STDERR



print STDERR "Arguments OK\n";
warn "Unable to find config file.\n";
die "Unexpected system error";
Lenwood Heath, Virginia Tech, Fall, 2004
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if Statementt
if (condition) {
statements;
} elsif (condition) {
statements;
} else {
statements;
}
if (not defined $ARGV[0]) {
die "Usage:\n\tpaint [COLOR]\n";
}
Lenwood Heath, Virginia Tech, Fall, 2004
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while Statementt
while (condition) {
statements;
}
$term = shift(@ARGV); $ln = 0;
while ($line = <STDIN>) {
chomp $line; $ln++;
if ($line eq $term) {
print "Term $term on line $ln.\n";
break;
}
}
Lenwood Heath, Virginia Tech, Fall, 2004
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for Statement
for (initialization; test; increment) {
statements;
}
for ($i = 0; $i < length($line); $i++) {
if (lc($char) eq substr($line,$i,1)) {
print "Character $char found.\n"; last;
} elsif (uc($char) eq substr($line,$i,1)) {
print "Character $char found.\n"; last;
}
}
Lenwood Heath, Virginia Tech, Fall, 2004
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foreach Statement
foreach $x (@y) { # execute for each element of list
statements;
}
foreach $color (qw/red green blue purple/) {
print "$color is a color!\n";
}
Lenwood Heath, Virginia Tech, Fall, 2004
21
Sample Script
#!/usr/bin/perl
$word = shift(@ARGV);
while ($line = <STDIN>) {
if (index($line,$word) > -1) {
unshift(@CONTAINS,$line);
} else {
unshift(@LACKS,$line);
}
}
foreach $line (@CONTAINS) {
print $line;
}
Lenwood Heath, Virginia Tech, Fall, 2004
22
Topics for Next Lecture
Subroutines
 Regular expressions
 Hashes
 File input/output
 File tests
 Invoking UNIX commands

Lenwood Heath, Virginia Tech, Fall, 2004
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